Temporary Armor Class Points
A wizard conjures balls of mud floating around him. As the bugbear attacks him, a ball meets each mighty blow of the morningstar, breaking, but deflecting it just enough for it to miss.
An elf takes a short while to free herself from distractions before a fight. The fresh focus will allow her to make a few physics-defying dodges before she loses it.
A muscular dwarf annoys his companions by pounding on his armor during a rest. He knows that well-maintained armor can be the difference between a strike being deflected aside or finding its way to his heart.
All these adventures make use of Temporary Armor Class Points, a rule designed to represent consumable methods of protection that focus on making attacks miss instead of soaking up their damage.
Temporary AC Points
A creature can be granted temporary AC points in a similar way to temporary hit points. If the feature granting them does not specify a duration, they last until a long rest. As with temporary hit points, you can only have temporary AC points from one source at a single time, unless stated otherwise.
When a creature that currently has any temp AC points would be hit by an attack, the creature loses temporary AC points equal to attack roll - AC + 1 and the attack misses instead.
If the creature doesn't have enough temporary AC points, or if the attack would hit or miss regardless of the target's AC (e.g. is a natural 20 or 1), the points are not spent and the attack follows as normal.
Effectively, temporary AC points are a consumable resource that can increase your AC against a single attack for just as much as is needed to make the attack miss.
Common modifications
Often, an effect will state that the temporary AC points granted by it also apply to Saving Throws. This works exactly as expected: if a creature that currently has any temp AC points fails a Saving Throw, it loses temp AC points equal to DC - Saving Throw and succeeds instead. If it doesn't have enough points or if the Saving Throw would automatically fail or succeed, no points are spent and the Saving Throw follows as normal.
Occasionally, effects create strongly quantified defenses, such as very big balls of mud. If an effect creates "X objects worth Y temporary AC points", you gain X times Y points from it. However, whenever you would spend some of these temp AC points, you must spend them in whole groups of size Y, rounding up. For example, if an effect grants you 5 orbs worth 4 points each, and an attack would cause you to lose 10 points, you lose 3 orbs (12 points) instead. By default, these objects still don't stack with points from other sources.
Sometimes, the temporary AC points granted by an effect are limited to X at a time. This means that if an attack would cause you to lose more than X points, it instead follows as normal and no points are lost.
Reasoning & Description
A flat AC boost may very well prove "useless" in a fight, if enemies roll either very low or very high. These points will nearly always be visibly useful without being overpowered (as you can grant much more points than "permanent" AC without upsetting the balance). They are meant to provide the satisfaction of a resource well used, while also adding another type of mechanical flavor to defensive features and spells.
I believe these points would feel more satisfying than temporary hit points, as they provide that "final touch" needed to avoid damage altogether, rather than simply soaking up some damage. Of course in some situations they may be better or worse, thus adding another layer of depth to the choice of spells and abilities.
While they require a little bit of calculations, they can replace multiple defensive effects (such as Mirror Image) with a single intuitive (and roll-less) mechanic, thus making the game a bit more streamlined.
Temporary AC points are usually best for avoiding singular blows, making them perfect in single combat or to soak up stray attacks. However, they may run out quickly after just a few attacks, making them much worse for crowded battles and somewhat more risky.
Variant Rules
The DM can allow points from multiple sources to stack at their discretion. It can be valuable especially when a character has two commonly used abilities granting them, such as Variant Dodge and Boon of Protection, so that the character can still get full value from both. By default, the points that can be refreshed more easily should be consumed first.
If you want to make temporary AC points even more satisfying, you can add the following modification: "When a creature that currently has any temp AC points would be hit by an attack, the creature can use its reaction (after the attack roll is made, but before damage is rolled) to use these points. The creature loses temporary AC points...". This prevents the points being wasted by attacks that would deal little damage, but the cost of a reaction provides balance.
The points could be made to also be spent if an attack hits by a lot, therefore being wasted. However, that defeats the main purpose, which is to make these points feel satisfying by rarely being useless.
Examples of Use
The following pages contain various spells and features that use temporary AC points. They should be relatively balanced, but as with all homebrew content, ask for your DM's permission before using them.
While the amount of examples provided here, along with temporary AC points requiring a little calculations, may seem overwhelming, remember that most characters will only ever encounter a few of these in their whole career.
Spells
Ablative Shield
1st-level abjuration (Sorcerer, Wizard)
Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell
Range: Self
Components: V,S
Duration: 1 minute
A layered magical barrier appears around you, deflecting incoming attacks. You gain 6 temporary AC points (which can be used against the triggering attack). These points last 1 minute or until depleted, and each time you are hit by a Magic Missile dart, you lose one of these points and do not take any damage.
Precognitive Defense
1st-level divination (ritual) (any class)
Casting Time: 1 bonus action
Range: Self
Components: V,S
Duration: Concentration, up to 1 minute
You glimpse into the near future and memorize the attacks that will be coming at you. You gain 8 temporary AC points, which last for the duration of this spell.
Orbiting Spheres
1st-level conjuration (Sorcerer, Wizard, Bard)
Casting Time: 1 action
Range: Self
Components: V,S
Duration: 10 minutes
You cause several orbs made out of clay, ice, some other brittle material, or whatever you're standing on, to levitate around you and deflect blows aimed at you. You gain 10 temporary AC points that last for the duration of this spell.
Defending Spirits
2nd-level abjuration (Cleric, Paladin, Druid, Ranger, Bard, Warlock)
Casting Time: 1 bonus action
Range: 60 feet
Components: V,S,M (a tiny offering for the spirits, such as a bit of incense, a pretty stone, or a bite of sweet food, which this spell consumes)
Duration: Concentration, up to 10 minutes
Small spirits (holy, natural, impish etc.) surround and protect the target at your request. For the duration, the target gains 5 temporary AC points at the start of each of its turns.
Reactive Shielding
3rd-level abjuration (any class)
Casting Time: 1 action
Range: Touch
Components: V,S,M (a small glass or crystal sphere)
Duration: 1 hour
You surround the target with a layer of protective energy that automatically concentrates itself to stop any threat. The target gains 15 + your spellcasting ability modifier temporary AC points for the duration of this spell. These points apply to Saving Throws and are limited to your spellcasting modifier + 1 (a minimum of 2) at a time.
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the target gains an additional 5 points for each slot level above 3rd.
Static Satellites
4th-level abjuration (Druid, Sorcerer, Wizard)
Casting Time: 1 bonus action
Range: Touch
Components: V,S,M (a small amber sphere, worth at least 20 gp)
Duration: 1 minute
You create 4 + half your spellcasting ability modifier orbs of electricity around the target. These orbs seek out attacks that would hit the target, especially ones nearest to its center. Each orb is worth 5 temporary AC points.
If the orbs manage to stop a melee attack that would otherwise hit the target of this spell, the attacker takes 1d4 lightning damage for each orb consumed. If the attack was made with a metal weapon, the damage increases to 1d8.
At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, you create one additional orb for each slot level above 4th.
Sustained Defense
5th-level abjuration (Bard, Sorcerer, Wizard, Warlock)
Casting Time: 1 bonus action
Range: 60 feet
Components: V,S,M (a small platinum sphere)
Duration: 10 minutes
You surround the target with two layers layer of regenerative magical power. For the duration, the target gains 8 temporary hit points and 4 temporary AC points (which also apply to Saving Throws) at the beginning of each of its turns. However, the temporary hit points can stack up to 24, and the temporary AC points can stack up to 12.
At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the target gains an additional temporary AC point for each slot level after 5th, and the stacking maximum increases by three times that amount.
Items
Armor Maintenance Kit
Common item
This kit costs 20 gp, and can hold up to 5 charges, which can be refilled for 2 gp each. It includes proper tools, special oils, spare pieces of metal and leather, and other resources used to keep armor in good shape.
You can spend an hour during a long rest maintaining your armor. To do so, you need this kit, along with tools appropriate to the armor type (usually Smith's Tools or Leatherworker's Tools). You must be proficient with these tools.
Maintaining armor expends one charge of this Kit and grants the wearer 2d4 temporary AC points. These points can stack with temporary AC points from other sources, and are used after them (and after points coming from a well-maintained shield).
Maintaining special or magical armor usually requires more specialized tools and resources. Your DM may choose them at their discretion.
Shield Maintenance Kit
Common item
This kit costs 20 gp, and can hold up to 10 charges, which can be refilled for 1 gp each. It includes proper tools, special oils, spare metal plates and leather straps, and other resources used to keep a shield in good shape.
You can spend 30 minutes during a short or long rest maintaining your shield. To do so, you need this kit, along with tools appropriate to the shield's type (usually Smith's Tools or Carpenter's Tools). You must be proficient with these tools.
Maintaining a shield expends one charge of this Kit and grants the wielder a single charge worth 1d4 temporary AC points. This charge can stack with temporary AC points from other sources, and is used after them (but before points coming from well-maintained armor).
Maintaining special or magical shields usually requires more specialized tools and resources. Your DM may choose them at their discretion.
Self-propelled Shield
Shield, rare
This gnomish shield has been equipped with miniature thrusters and a semblance of intelligence to understand its job.
At any moment during your turn (no action required), you can press a switch and cause the shield to detach itself from the arm bracer and start flying around you, intercepting attacks. You no longer count as wielding a shield, but gain 10 temporary AC points that last for a minute, until they are depleted, or until you recall the shield on your turn (no action required). The shield automatically reattaches itself when the points are depleted or a minute has passed. If your hand is occupied, it falls to the ground at your feet instead, requiring a bonus action to reattach.
You can use this ability once. To recharge it, the shield must spend at least an hour near a fire or another strong source of heat.
Elven Buckler
Shield, any rarity
On rare occasions, elven warriors wield a light buckler in their off-hand. These bucklers require great reflexes to use, but can prove far superior to a heavier shield, as long as the enemies don't overwhelm the wielder.
You need a Dexterity score of at least 15 to use this buckler. While wielding it, you do not gain the normal benefit of wielding a shield. However, at the start of each of your turns, you gain temporary AC points. Their number depends on this item's rarity - 4 for Common, 6 for Uncommon, 8 for Rare, 10 for Very Rare and Legendary. The Legendary variant also provides a permanent +1 to AC while wielded.
These points stack with points representing quick reflexes, such as those from the Reflexive Defense or Focused Mind fighting styles.
Gambler's Bracelet
Wondrous item, rare (requires attunement)
This golden bracelet is filled with a strange essence of luck, allowing you to improve your odds, but also increase the stakes.
While wearing it, you can reroll any 1 you roll on a d20 and use the new roll, but if you roll an odd number, it still counts as a natural 1.
When you are targeted by an attack, but before you know the outcome of the attack roll, you can use your reaction to grant yourself temporary AC points equal to your Charisma modifier (minimum 1). If you take damage from an attack while you have any of these points, you increase the damage by the number of points remaining and lose the points. Otherwise, these points last for a minute or until depleted.
You can also use this ability as a bonus action.
You can use this ability once every short rest.
Abilities & Feats
Fighting Styles
The following are fighting style options that can be chosen by any class that allows a choice of a fighting style. If you take both of these styles, the temporary AC points granted by them stack with each other.
Reflexive Defense. As long as you are not wearing heavy armor, you gain 2 temporary AC points at the start of each of your turns.
Focused Mind. Whenever you roll for initiative, you gain 8 temporary AC points that last for a minute.
Variant Dodge
If you prefer to defend yourself with quick reflexes rather than immovable brawn, you may replace your standard Dodge action with the Variant Dodge.
When you use the Variant Dodge action, you gain 5 temporary AC points that last until the beginning of your next turn. These points stack with points from other sources and are used first.
This version of the action fits classes such as Rogue and Monk thematically. Mechanically, it is stronger against single enemies, but weaker against multiple enemies (or enemies with multiattack), making you better at skirting around the main battlefield than holding your ground or charging right in.
A similar change can be applied to Way of the Kensei Monk's Agile Parry, making it provide 5 temporary AC points instead of a flat 2 AC.
Feats
Limb defense
Prerequisites: at least two upper limbs
You have learned to take blows to your limbs, protecting vital organs. You treat your hit points as if they were also temporary AC points limited to 3 at a time. This happens after regular temporary AC points were used or bypassed. For example, if your AC is 16, and an enemy makes an attack roll of 18 against you, you lose 2 hit points, but the attack misses.
At level 6, the amount of hit points you lose is doubled (but they still provide the same amount of temporary AC). At level 11 it is tripled instead, and at level 16 it is quadrupled instead.
This feat would also make a great racial ability for a race with long or resilient limbs.
Turn the Tide
Prerequisites: none
Your willpower is iron and your heroism cannot be denied. Whenever you recover hit points while below half of your maximum hit points, you also gain temporary AC points equal to your proficiency bonus. If you are at 0 hit points, you gain temporary AC points equal to twice your proficiency bonus instead. You must finish a long rest to benefit from the increased effect again, otherwise you benefit from the base effect.
Other
Boon of Protection
This can represent a Warlock's Pact Boon, or perhaps a blessing of a deity.
You are blessed with supernatural luck, protecting you from enemy blows. You gain 1d6 temporary AC points on a short or long rest.